Gas cell for the optical analysis of gases

ABSTRACT

A measuring cell for a gas analysis spectrometer has an inner chamber ( 23 ) for a sample gas to be analyzed and an inlet ( 21 ) and an outlet ( 22 ) which are connected thereto. A traversing optical path for a measuring beam ( 14 ) is formed in the inner chamber ( 23 ). The measuring cell is tubular, the inlet ( 21 ) and the outlet ( 22 ) are arranged at opposite ends, and the inner chamber ( 23 ) of the measuring cell has a cross-sectional shape that is monotonic over the length of the tube and which has an oval-shape at the start, which disappears toward the end. That special shape results in fast gas exchange and thus high dynamics, even with larger measuring cells, which have high sensitivity due to the long optical paths thereof. Two characteristics which until now appeared to be conflicting are thereby combined.

The invention concerns a measuring cell for a gas analysis spectrometerwith an inner chamber for a gas to be analyzed, an inlet and an outlet,wherein a traversing optical path for a measuring beam is formed in theinner chamber.

The optical analysis of gases is applied widely in various areas oftechnology. Special requirements are demanded by exhaust gas measurementapplications for internal-combustion engines. Due to increasingly strictexhaust gas regulations, not only is a high level of sensitivityrequired to achieve a low detection threshold, but also a high timeresolution to ensure a sufficiently good dynamic response of themeasurement, in particular, with respect to non-stationary operatingstates of internal-combustion engines. This results in a conflict ofobjectives between detection sensitivity and the time resolution of thesystem. In such measuring cells for optical gas analysis devices, thedetection sensitivity depends on the optical path length that themeasuring beam travels through the gas to be analyzed in the measuringcell. This path length, in turn, depends on the inner chamber volume ofthe measuring cell and the guidance of the measuring beam. However, thetime resolution that is decisive for the dynamic response directlydepends on the time needed to replace the gas to be analyzed in themeasuring cell. It is important that the gas is replaced in itsentirety. Increasing the volume of the measuring cell therefore has thedisadvantage that, while other parameters remain constant, the timerequired to completely replace the gas increases, causing the timeresolution and therefore the dynamic response to decreasecorrespondingly.

Various approaches for increasing the quality of the measurement areknown from prior art. In many measuring cells, attempts are made toincrease the detection sensitivity for a constant cell volume byoptimizing the optical path. U.S. Pat. No. 5,440,143 A1 describesattaching a special mirror system onto an otherwise standard measuringcell with a square cross-section, which produces a multiply folded andtherefore extended optical path for the measuring beam. Disposingmultiple measuring cells one behind the other so that the measuring beamis first guided through a first measuring cell and then through another,is known from US 2007/0182965 AI. A universal measuring cell foradapting the length of the optical path is known from JP 10/062,335 A,wherein the cell is constituted as two telescopic partial bodies.

An alternative approach has tried to influence the flow of sample gaswithin the measuring cell (DE 103 18 786 A). In such a measuring cell,however, relatively large “dead zones” are formed, which increase theexchange time and worsen the dynamic response. As FIG. 7 schematicallyshows, in a measuring cell (9) according to prior art, swirling (91) ofsample gas in the measuring cell causes formation of dead zones in whichmolecules of the sample gas can dwell for a comparatively long time,preventing fast exchange. As the concentration of the supplied samplegas (90) changes, the previous concentration is still partly present sothat the new concentration value can only be correctly determined oncethe gas in the dead zones has also been exchanged. The resulting timedelay causes carryover (concentration carryover), which in turn resultsin a long response time of the measuring cell and therefore of theentire measuring system.

The object of the invention is to create an improved measuring cell witha better dynamic response.

The inventive solution is a measuring cell with the characteristics ofthe independent claim. Advantageous embodiments are the subject of thedependent claims.

In a measuring cell for a gas analysis spectrometer with an innerchamber for a gas to be analyzed (sample gas) and an inlet and an outletconnected to it, an optical path traversing the inner chamber is formedfor a measuring beam, wherein according to the invention, the measuringcell is constituted as a tube with the inlet and the outlet at oppositeends, and its inner chamber has a cross-sectional shape, which extendsmonotonically over the length of the tube, with an ovality at the start,which disappears toward the end.

Some of the concepts and terms used are explained below:

Inlet describes a facility through which sample gas can flow into theinner chamber of the measuring cell. Correspondingly, outlet describes afacility through which it flows out.

The beginning of the measuring cell describes the region where the inletis positioned. Correspondingly, the end of the region is that whichleads to the outlet.

Monotonic means a change that occurs in one direction only. An ovalitythat decreases monotonically along the length of the tube thereforemeans that at no point does the ovality increase along the length of thetube, not even intermittently.

The inventive measuring cell has a shape that is optimally adapted tothe formation of a vortex at the inlet of the sample gas and thetransformation of the vortex as it moves toward the outlet and in such away that the flow of gas that moves from the inlet to the outlet fillsthe entire cell volume along a direct path. The emphasis here is on adirect path, i.e. secondary curls or other fluidic figures do not haveto be formed to exchange the gas in remote zones (dead zones). Indeed,the inventive shape avoids the existence of such dead zones, resultingin particularly fast gas exchange due to the exchange along a directpath.

The invention has recognized that the dynamic response of the measuringcell can be improved not only with a particularly small size of the cellvolume but, in contrast to previous attempts in prior art, also with alarger size of measuring cell having a special shape. This special shapeis provided by the ovality on the inlet side, which disappears towardthe outlet. As has already been mentioned, this special shape allows aparticularly fast exchange of gas and produces the desired improvementin the dynamic response. This invention therefore no longer relies on anespecially small size of measuring cell, enabling the measuring cell tobe larger and therefore more robust. This lengthens the optical path forthe measuring beam and these good optical conditions improvedetectability of the measuring cell. The invention therefore achieves acombination of advantages with respect to improved dynamic response andimproved detectability. It achieves this in a surprisingly simple way,namely solely by ingenious shaping of the measuring cell. There is noexample of this in prior art.

To reliably achieve favorable vortex formation even as the sample gasflows in, the inlets are preferably disposed in the tube casing.Disposing them thereby in the region of the start of the tube has theadvantage, compared to positioning on the start end face, that reliableand fluidicly advantageous main vortex formation can be achieved. Thisparticularly applies when the inlets are disposed diametrically oppositeeach other, and offset with respect to the central axis of the tubeshape. This not only applies if two inlets are provided but also if morethan two inlets are provided: in this case, they should be disposed insuch a way that the sample gas initially flows into the tubetangentially. With this configuration, the inflowing sample gas can beinduced to swirl. This results in stabilization of the flow and ensuresthe desired penetration of the entire inner chamber volume with the mainvortex.

The outlets for the exiting sample gas are preferably constituted withan axial component. This is understood to mean that the outlets have anangle of maximum 30° with respect to the tube axis. Disposing them onthe casing allows the mirror for the measuring beam to be disposed inthe center. In this way, the end region can be optimally used forgenerating the optical path for the measuring beam. Furthermore, thisoutlet configuration has the advantage that unimpeded exit of the gascan be achieved due to the considerable tangential component. Theoutlets are preferably tapered. This is understood to mean that at theirstart, i.e. in the region of their entry, they have the largestcross-section, which successively tapers the further it is from theinner chamber. It has been shown that a particularly good dischargecharacteristic from the inner chamber into the outlet of the sample gascan be achieved in this way, particularly with respect to the paucity orabsence of reflections and the vortex or antivortex caused by them.

Preferably, the ovality in the region of the outlet disappearscompletely. This is not absolutely necessary, a slight ovality (comparedwith the inlet) can remain. Preferably, the shape of the tube of themeasuring cell in the region of the outlet is circular. Advantageously,it is already circular at some distance (up to ⅓ of the total length ofthe tube) from the position where the outlet is disposed. In thisconnection, the cross-sections preferably have substantially equalsurface areas despite being different in shape, wherein by“substantially” a deviation of no more than 15%, preferably 10% isunderstood.

In most cases, the outlet will be disposed in the end region of thetube. However, this is not absolutely necessary. Therefore, anadditional element can be provided in addition to the tube, which has across-sectional shape that is inverse with respect to the tube. It isdisposed in such a way that the non-oval side of the measuring body(i.e. its end) is connected to the correspondingly shaped beginning ofthe additional element, and the additional element changes to becomeoval along the length of the tube. This intermediate element thereforeprovides a sort of continuation of the original measuring length. Thisis especially suitable for the detection of sample gases in especiallylow concentrations.

The invention is explained below using the included drawing, which showsan advantageous embodiment.

The drawings show:

FIG. 1 A schematic representation of a measuring device with aninventive gas cell;

FIG. 2 A representation of the gas cell showing the beam path;

FIG. 3 A view from above onto the gas cell without its inlet and outlet;

FIG. 4 A sectional view of the gas cell;

FIG. 5 An alternative embodiment of the gas cell;

FIG. 6 An exploded view of the gas cell according to FIG. 2; and

FIG. 7 A conventional gas cell.

The invention is explained using the example of an FTIR spectrometer.FTIR stands for Fourier transform infrared spectroscopy. Such devicesare known from prior art and will therefore be only briefly explainedwith reference to FIG. 1.

An infrared light beam 10 (IR beam) from a source 11 for infraredradiation is focused onto an obliquely disposed beam splitter 12 of aninterferometer, which is collectively designated by reference numeral 1.The IR beam 10 is divided into two components 10 a and 10 b, of whichcomponent 10 a is reflected by the beam splitter 12 to a fixed mirror 13a, and component 10 b is allowed to pass through to a movable mirror 13b, whose distance from the beam splitter 12 can be altered (symbolizedby the dashed double-headed arrow in FIG. 1). The partial beams 10 a, 10b reflected back by the mirrors 13 a, 13 b, interfere at beam splitter12 and are together radiated as IR measuring beam 14 into a gas cell 2.

The gas cell 2 is the actual measuring cell. Conventionally, it isconstituted in the shape of a cell or vessel (cf. FIG. 7). It has anelongated basic body 20 with an inlet 21 at one end and an outlet 22 atthe other end. The gas to be analyzed flows through the inlet 21 intothe basic body, fills the latter and flows out again through the outlet22. While the gas dwells in the basic body 20, the gas is irradiated bythe measuring beam 14. Depending on the composition and concentration ofthe gas in the gas cell 2, different components of the spectrum of themeasuring beam 14 will be absorbed and the remaining component that isallowed to pass through (transmitted) is projected onto a detector 15.

Detector 15 is an MCT semiconductor detector, which converts the changein photon intensity into an electrical quantity. However, a photodiode,a bolometer or the like can also be used. The signal measured bydetector 15 is guided to an analog/digital converter 16. Theinterferogram 18 can be displayed on a suitable display device. Then,what is now a digital signal is processed by a transformation element 17by means of fast Fourier transform (FFT). It is constituted to generatea spectral representation 19 from the interferogram provided by theanalog/digital converter 16 in a known way and to display it.

The functional and structural configuration of gas cell 2 is shown inFIGS. 2 to 6. As FIG. 2 most clearly shows, the gas cell has anelongated, round hollow basic body 20 with a double-entry inlet 21 atone end and a double-entry outlet 22 at its other end. The basic bodyhas a cavity 23, which is delimited by a casing 27. According to a coreelement of the invention, the cross-section of the cavity 23 in thebasic body 20 is not constant but changes continually from inlet 21 tooutlet 22. According to the invention, the shape of the cross-section ofthe cavity 23 has been chosen such that the cross-section is oval atinlet 21 and this ovality is increasingly reduced toward outlet 22,until it practically disappears completely in the region of outlet 22,i.e. there, the cross-section is practically circular. This permits useof a round mirror 32 in the outlet region to reflect the measuring beam14 and a polygonal mirror 31 in the region of the inlet cross-section.The mirrors 31, 32 have the same radius of curvature.

The inlets 21 are disposed on the basic body 20, diametrically oppositealong the longer axis of the oval, with a small offset in oppositedirections (less than one tenth of the size of the width of the basicbody 20 in this region) relative to the center axis 24 of the basic body20. In this way, it is ensured that the sample gas flowing in quicklyfills the oval-shaped cross-section. An intended asymmetry is achievedby this offset with which the flow in the cavity 23 takes a preferreddirection so that a defined vortex can form, which ensures fast mixtureat the beginning and during continued flow of the sample gas towardoutlet 22. Because of the tapered cross-sectional shape, the vortexalong the path to the outlet 22 gradually turns into a circular vortexand its peripheral speed slowly decreases. At the outlet end, theoutlets are disposed diametrically opposite and oriented in such a waythat they are tangential to the direction of flow (symbolized by arrow5) from inlet 21 to outlet 22 and form an angle α of approx. 25° withrespect to the center axis 24. In this way, the sample gas can exit thegas cell 2 via the outlets 22 in a way that is favorable to the flow.

The beam guidance with the IR source 11 and the detector 15 and theinstallation location with reference to the gas cell 2 are shown in FIG.4. The measuring cell 2 represented in the embodiment is 16 cm long andhas a 7.5-cm diameter. A floor-sided pot 4 is provided beneath theactual gas cell 2, in which the IR source 11, the detector 15, and theinterferometer 1 are disposed. The IR source and detector can also bedisposed externally, in which case corresponding access openings for theinflow and outflow (represented by a dashed line) would have to beprovided. They radiate through openings located at the edge of thepolygonal mirror 31 (see reference figure 35 in FIG. 3). Taking intoconsideration this surface intended for the beam entry and exit, thepolygonal mirror 31 forms an envelope that is elliptical. The gas cell 2is closed at its top end by a cover 26. Further, the round mirror 32 isdisposed on the inside of the cover 26 so that it faces the polygonalmirror 31. The round mirror 32 is configured as a double mirrorcomprising two parallel concave mirrors 32 a, 32 b. Their radius ofcurvature is identical and dimensioned such that their focal points arelocated on the surface of the opposite mirror 31. Mirror 31 is alsoconcave, wherein its focal point is aimed exactly onto the center of thetwo concave mirrors 32 a, b. This results in a multiply reflected,fanned out light path for the measuring beam 14, which forms astationary beam pattern in the two concave mirrors 32 a, b, and a beampattern on the polygonal mirror 31 that moves slightly each time itreflects back and forth. In this way, both mirrors 31, 32 areilluminated fully for the measurement. All the input light is reflectedfrom one mirror 31, 32 to the other 32, 31, so that there is practicallyno loss. The fanning out with multiple reflection produces a light paththat is a multiple of the actual overall length of the gas cell 2 (seeFIGS. 2 and 4).

Several advantages are achieved in this way. On the one hand, sample gasflowing in at inlet 21 is immediately caught by measuring beam 14, whichresults in a very fast response time. The sample gas is measured beforeit even has time to mix with the old gas still present in gas cell 2. Asa result, changes to the composition and/or concentration in the samplegas are visible practically immediately. The invention has alsorecognized that the claimed cross-sectional transition shape not onlyprovides advantages in terms of minimizing the internal volume of thegas cell 2 but is also provides favorable conditions for flow. When thesample gas flows in, a vortex is formed, which more or less fills theentire cross-section in the inlet area, and changes shape along its pathto the outlet such that it acquires an increasingly circularcross-section. The invention takes advantage of the behavior of themeasuring gas vortex by adapting the cross-sectional shape of the gascell precisely to this change in shape, thus having a cross-sectionalong the entire length of the gas cell that is entirely filled by theflow. This effectively reduces the “dead zones,” which are critical tothe response and precision. Because the gas exchange in the gas cell isfaster than the measurement of an interferogram, a maximum dynamicresponse is achieved.

The long light path results in a high level of sensitivity. The lightfan produced between the polygonal mirror 31 and the circular mirror 32is optimally adapted to the cross-sectional shape of the inner chamber.This results in practically the entire inner chamber being irradiatedand, because of the complete filling with the flow described above,quickly being filled with the entering sample gas (without formation ofthe disturbing dead zones known from prior art). The wide fanning inconjunction with the flow pattern produced by the special shape ensuresa fast response. In this way, the inventive gas cell can provide twoessential advantages at once.

To further increase the sensitivity while maintaining the advantageousdynamic properties, an alternative embodiment is possible. It has anadditional element 6, which is directly connected to the gas cell 2. Inthis case, the cover 26 of the gas cell 2 is eliminated so that,together with the additional element, a large uniform cavity 23′ isproduced. The shape of the cavity in the additional element 6 isinverse, i.e. circular where it forms a connection with casing 27 of thegas cell 2 and oval at the outside end. The additional element 6 ispreferably constructed identically and connected to the coverless gascell 2 in a “back-to-back” configuration. Inlet 21 is located at thebase of gas cell 2 and inlet 22′ is located at the other end at theadditional element 6. With this configuration, the sensitivity can bealmost doubled, wherein the advantageous shape of the gas cell 2 isretained due to the mirrored shape of the cavity of the additionalelement 6.

1-8. (canceled)
 9. A flow-through measuring cell for a gas analysisspectrometer, the measuring cell comprising: a casing defining a tubularinner chamber for a sample gas to be analyzed, said tubular chamberhaving an outer surface, a first end and a second end; an inletcommunicating with said tubular chamber at said outer surface andproximate said first end, said inlet having a tangential component; anoutlet communicating with said tubular chamber at said outer surface andproximate said second end, said outlet having an axial component,wherein said tubular chamber has a substantially oval cross-sectionalshape proximate said inlet, which decreases monotonically along a lengththereof and substantially vanishes proximate said outlet; a firstconcave mirror disposed at said first end of said tubular chamber; asecond concave mirror disposed at said second end of said tubularchamber, wherein said first and said second mirrors induce a multiplyreflected, fanned out optical path for a measuring beam in said tubularchamber.
 10. The measuring cell of claim 9, wherein said inlet isdisposed in said casing of the measuring cell.
 11. The measuring cell ofclaim 9, wherein said axial component of said outlet is disposed in sucha way that said outlet forms an angle of no more than 30° with a centeraxis of the measuring cell.
 12. The measuring cell of claim 11, whereinsaid outlet has a cross-section that is tapered towards an outside. 13.The measuring cell of claim 9, wherein said tubular chamber has asubstantially circular cross section proximate said outlet.
 14. Themeasuring cell of claim 9, wherein said cross sectional shape of saidtubular chamber at said inlet and said outlet differ but have asubstantially same surface area.
 15. The measuring cell of claim 9,further comprising an additional element which is connected to an outletend and has a cross-section progression, which is inverse with respectto said tubular chamber.